Warning: Turn down gain before switching input

The demo of an early teletype machine produces spectacular, if unintended, results

The demo of an early teletype machine produces some rather spectacular, if unintended, results

Years ago I worked for a large manufacturer that was a leader in designing and building teletypewriters. For the younger set, a teletypewriter had a keyboard and printer with no computer between them. It had serial input/output capabilities allowing it to communicate with remote similar machines. One large user of such machines was Western Union for their telegraph service.

At the time of this story, these units were mostly mechanical plus a few switches, solenoids and relays; no electronics. They were mostly assembled from parts punched out of steel. Data moved into and out of them at an astounding 10 characters per second.

Many of this company’s products were used by the government and military, so we had some impressive test equipment to verify that the products would work over the needed environmental range. For instance, there was a walk-in chamber capable of greater than 120F at greater than 95% humidity, another capable of simulating an elevation of 30,000 ft. (Note: All numbers are subject to poor memory.)

Management was so proud of the facilities that they held an annual open house for employees and family.

One year, they added a shake table to the test equipment facility and wanted to showcase it at the open house. Its purpose was to vibration-test products over a frequency range from 0 to 5000 Hz. For example, you might want to test your product to make sure that it didn’t rattle apart when you installed it in an airplane and the engines created a beat note at say 10 hz.

If I remember correctly, the shake table was built by a company then called Ling-Temco-Vought. It consisted of an electronic amplifier and a loudspeaker-like voice coil, and it was big. The final amplifier, which was visible through a window in the cabinet, consisted of two vacuum tubes each about the size of a 5-gallon bottled water tank. These were air cooled, and glowed dull red-orange in operation. The voice coil looked like a large kettle drum. It was five or six feet in diameter, water cooled and capable of shaking a load on the order of 100 pounds that sat on the “drum” surface.

For the open house, someone decided to have the shake table introduce itself to the visitors as part of the demonstration. It could do this since the speaker assembly made audible sounds as it shook the payload. They also decided that the unit under test for the demonstration should be a brand new teletypewriter that was under development.

This "new" product was an ASR-33, pictured here:

The shake table had two sources of test frequencies. An internal oscillator could be used to provide the needed 0-5000 Hz at controllable amplitude. There was also an input jack allowing an external source of oscillation.

For the demo, a tape recorder was hooked to this external input with the recording, “I am the Ling-Temco-Vought model xxxx and I test …………” After the tape recording introduced the shake table, the operator was to switch the input to the internal oscillator, which would run through a pre-programmed frequency profile. Of course, this all worked well in rehearsals. On the evening of the open house, with about 50 people gathered around, the operator started up the shake table and the tape recorder. The Ling-Temco-Vought successfully introduced itself. Then the operator threw the switch to activate the internal oscillator.

However, he forgot to turn down the gain control before he did that. You know that little static pop that happens when you switch from one audio source to another? Magnify that by, oh quite a bit, and you can probably visualize what happened: The shake table voice coil gave a mighty pop, and the teletypewriter which was operating during the demo was destroyed. Steel parts were actually launched through the plastic cover, and the cover was broken free from the base of the unit. It was very impressive, and needless to say, the demo was finished for the evening.

The next day a warning sign appeared on the front panel: “Turn gain down before switching input.”

Charles Glorioso has a BSEE from Purdue and an MSEE from the Illinois Institute of Technology. He has over 40 years experience in electronics design and management for industrial and consumer products.

Some engineering took some doing. A rugby feed from UK to New Zealand first time had the players followed some distance by the color of their uniform. Group delay equalizers were needed. The feed from the US had to go through a Rank analog TV standards converter which took 3 racks and used crystal delay lines for interpolation. Both are probably no longer documented designs, but they are how we got to where we are now.

Sometime back, more years than I care to consider, I was a young engineer part of a team developing a sonobouy receiver for the Navy's S3A aircraft. The specifications were tight enough that we had an interesting time with the crystal filter designs. By the end of the project these and the entire RF chain were my sole responsibility. I'd managed to specify some rather nice filters and they worked nicely.
To prove the filters I'd developed a nice test board and some procedures that got me past some of the limitations of the then current spectrum analyzers that plagued anyone without a tracking signal generator. So we really had good equipment to deliver to the Navy.
So we built the prototype. Somewhat predictably for the era we found a problem or two and generated tiny work arounds for them. Then we ran preliminary tests and it worked like a champ.
Finally we entered the qual testing phase. This involved a large shake table, LTV I believe. We sailed through the shock and vibration tests. Unfortunately these were not the last tests to be run on that ill fated receiver. The test technician at the shake table facility accidentally pulled the signal source before cutting power. The predictable rather extreme shock rather rattled the receiver's brains. Some of the 31 receivers failed on the spot. So I had to pull all the boards and find the good ones and replace the bad ones. The failures often were not "instantaneous", which if memory serves led to replacing ALL the boards. I watched one fail under retest. It looked fine one moment and showed garbage moments later as another crystal departed from one of its mounting leads.
This would have been late in the ASR33 era (about 1970). So I guess the shake table people took a long time to learn about click and pop filters. I feel your pain, Charles.
{^_^}

There once was a Star Trek game in BASIC that depended on the Model 33's timing of the auto carriage return / line feed. The PRINT statement centered the title words "S T A R T R E K" but left out one of the characters, printing it as the 73rd character of the 72-character line. If the ASR was set up for auto carriage return, it did a return before a line feed, and the missing character was printed as the print head returned, before the line feed (the original bi-directional printer!). Some software was set up to put out two CRs before the LF to make sure the print head was all the way back to the left.
If the auto carriage return was not set, all the characters after 72 piled up on the 72nd character.

This brings back many fond and not so fond memories. I hated being a teletype repairman in the US Navy, but I like typing on them. I was a Radioman and we used mostly the model 28s. When I got out of the Navy I found an old Kleinschmidt table top teletypewriter. It was something to behold. I used it on Ham Radio for many years before going electronic. What Fun!

I didn't get to play with the ASR, but I certainly had to run quite a few shake tests on vehicle bodies and parts of cars, all using those old tube type amplifiers. They were indeed very impressive, and they could produce fast transients, with far more than their rated output, for two or three seconds, which was plenty of time to do a whole lot of damage. So we would start with the gain control at minimum every time. Of course, some of the testing was done with a feedback controller, either holding constant force with a force sensor in the drive link, or with constant velocity, using a displacement sensor, and recording the force, which drops as resonance is approached. That can be exciting, I have seen a pickup truck broken just by driving it at the resonant frequency for a few minutes. It was quite a show.

Oh, man! I remember using the old mechanical Teletype machines when I was in the USCG. It may be apocryphal, but when we were in tech school, we were told that the designer of the model then in common use (I don't remember the number maybe the 23?) was a mechanical genius who eventually committed suicide. Watching those machines work was absolutely fascinating, and makes me even more appreciative of modern comm devices. I also remember acoustic-coupled 110 baud ASR33 machines talking to our IBM 370 many years ago. Now I complain if my connect speeds are lower than 1.5 mbit!

All great comments about ASR33 Teletypes, and early computer usage of them. (By the way "Teletype" is a registered trademark of the company by that name. Like Kleenex, they had such a high market share that they almost lost their trademark due to common usage.)
Those Model 33 devices were designed for light duty usage in Telex and TWX (Telegram) usage. But they hit the market at exactly the right time for use in the first time sharing computers, as described by Mtripoli. As a result they were a much more successful (profitable) product than planned.
Charles

My father was a communicator in Civil Air Patrol (CAP), the USAF's poor relation, which always had to make do with hand-me-down equipment. Over the years, we had several TTYs in the basement; their racket was a familiar, even comforting sound.
Since the machines had attached punched tape readers, Dad created several loops of tape with routine message headers, etc., with the drive holes cut out at points where he would insert message-specific information.
When the machines wore out beyond repair, upon decommissioning, he'd give them to me to tear apart. I had a wonderful collection of micro-switches and other state-of-the-era parts.
Lance ==)--------------